Well you're thinking like I'm thinking.
Will he be the missing piece of the puzzle for United or another bang average high earner with a good agent.
TBF, i believe that it's the wrong to think of transfers as the "missing pieces of a puzzle" because the puzzle (or whatever you wanna call it) is - or at least
should be - a living and breathing organism, always ready to adapt, evolve and expand. You pay money in the market to add qualities and if you fork out a king's ransom, you better make it worth the purchase. That's where United have been failing at for the last decade or so.
So, to answer your question, i think he's compatible with what we already have. He's a neat and tidy midfielder: Good positioning and concentration, sound defensively and able to receive the ball in the build-up and either pass or carry it to his nearest teammate. He can be a part of a midfield partnership that will allow Bruno to do reassume his "free roaming" role as an attacking midfielder. There's that.
But for the salary he's currently asking, we should be asking whether we're getting good value for our money. When i look at this United side, the midfield in particular, i see a team that can't control the rhythm of matches or force its tempo to its opponents. We constantly stumble with the ball at our feet because we're slow when we need to be precise and rush when the situation demands of us to be quick. Thus, we rely on moments of brilliance, more often than not, to get the job done. These are chronic issues.
Will putting Rabiot on 250 grand p/w improve us in that area? I doubt it. He can help ETH (or probably his successor) revert to what has served us in the past and give us a fighting chance at a top-four finish. But when that dreadful moment comes again, when we have to push for bigger things? No, i don't think he's the answer to that. By then, he'll be another high earner that we would be better off without.
For what it's worth, i didn't think Casemiro was the answer, either, although this place seems enamored with midfield "destroyers" and "workhorses". Fergie won his last two PL titles with Carrick and Scholes/Giggs in their mid/late 30s playing in the midfield. If you think about it, he never actually cared for such profiles (for those lucky enough to have watched him, Keane was so much more than a defensive midfielder). Why? Rhythm and tempo...